Cycle World Service

CYCLE WORLD SERVICE

Mikuni pilot jets

In our carb tuning experiments we’ve discovered that pilot jets for different Mikuni carburetors aren’t all interchangeable. That’s not so bad in itself, but the jets look almost identical, and some dealerships have different types mixed together in their jet boxes. If you plan on changing Mikuni pilot jets, you should be able to distinguish between them.

The most common type is the VM22/210 jet, and is used in all VM type slide throttle Mikunis from 20 to 44mm throat size. (These are the carburetors used on most dirt bikes and pre-emission era street bikes.) The VM pilot jet is the one most stocked at dealerships, and can be distinguished by the location of its metering orifice, near the center of the jet, next to the threads but below the air bleed holes.

The other common Mikuni pilot jet is the BS30/96 type, used in most BS Mikuni constant velocity carbs, the ones that come on recent street bikes. A BS pilot jet looks very much like a VM pilot, but the small orifice hole is at the very tip of the jet, in the pointed end away from the screw threads.

The final configuration of Mikuni pilot jet you’re likely to encounter is the N151.067. It’s used in some BS constant velocity carbs on Kawasaki models, and is identical to the standard BS pilot jet, but with the air bleed holes in the side deleted. The air bleed holes haven’t been required on recent model BS carbs, but were left in all BS30/96 pilot jets to maintain interchangeability. Cost finally got the better of customer convenience, and the N151.0567 jets were made without the air bleed holes to save a drilling operation.

As far as what pilot jets to use where, it’s always safe to use the jet that was intended for a given carb type. VM jets don’t work in BS carbs (they give a richer mixture than the orifice size indicates), and BS jets can’t be used in VM carbs. The only acceptable interchange is BS30/96 pilots for the NI 51.0567 jets, which is fortunate as the BS30/96 jets are more broadly distributed. Your dealer can order them from Mikuni distributors Sudco, Gem Products, or Rocky Cycle.

Revs hang up

My 1981 GS450L Suzuki has approximately 600 mi. on it and runs great. However, the rpms do not drop off rapidly when I let off the throttle. In fact the engine speed stays the same long enough that when I’m shifting I need to coast for a second or two to allow the rpms to drop so I can shift smoothly.

The mechanicals of the bike still seem tight because it is so new. But it seems that the rpms should fall back to idle speed as soon as I let off the throttle. What could be the problem?

Theodore Havens

Battle Creek, Mich.

As a first step to solving your problem, you should check and see if it has a simple mechanical cause. Is the throttle cable or linkage binding? Turn the throttle and watch and see if the actuating lever on the carb body returns immediately when you release the throttle. This lever controls the throttle but ter fly s, and if it isn’t returning quickly the problem is in whatever is causing it to hang up.

If it does return immediately the carbs are probably not synchronized, and the idle speed is almost certainly set too high. Constant velocity carbs have both a mechanically controlled throttle (the butterfly valves mentioned above) and a vacuum controlled slide that controls the venturi height. When you back ojf the throttle to shift gears, the butterfly valve closes as far as the idle adjustment screw will allow, but the vacuum controlled slide doesn’t have time to close. This combination gives less flow restriction than if both the butterfly and slide were closed, so there is some tendency for revs not to drop as fast with CV carbs as with slide throttle carbs. This tendency is minimized with the proper idle adjustment, which will only work if the carbs are synchronized. So synchronize the carbs (follow the procedure in the Suzuki shop manual or have a dealer do it) and set the idle rpm to the Suzuki recommendation, and your problem should be solved.

O-ring chain life

t traded a chain drive CB750 for a shaft drive GL1000 in 1976 to get away from the messy chain.

However, the poor handling of the GL has gotten to me over the years and now I’m thinking of going back to a chain drive bike (CBl 100F). Do the O-ring chains require as much attention as the old non-O-ring chains and are they messy?

Walt Taylor Agoura Hills, Calif.

Current O-ring chains are far more durable and require less maintenance than the chains available in the early 1970s. A good O-ring chain has a service life of8,000 to

12.000 mi. or more. Chain adjustments are required less frequently, perhaps every

1.000 mi. instead of every 200. Some lubrication is still required, so an O-ring chain isn’t as clean as an enclosed shaft drive.

Suzuki Triple pipes

I’m the proud owner of a 1977 Suzuki GT750B. In 1978, I installed a set of J &

R power-pipes, and now, after

40.000 mi., they’re starting to wear out. Is J & R still producing these chambers? If > not, could you recommend other pipes for the Triple?

While speaking of expansion chambers, I’ve seen several GT550s and 750s with 3-into-l pipes. Do they make any power? Does anyone still make 3into-1 chambers?

Also, I’m interested in lacing a 16 in. rim to my front hub. Can you recom-

you mend a source for the rim and spokes? Bob Millett Framingham, Mass.

J& R no longer makes pipes for the GT750. However, Bassani Mfg. (3726 E. Miraloma, Anaheim, Calif 92806, phone (714) 630-1821) lists GT750 pipes in its catalog.

Strader Engineering made 3-in-l exhausts for Suzuki Triples, but, as far as we can tell, the company is no longer in business. The pipes themselves gave some power increase over stock, but not as much as individual expansion chambers. The main benefits of the 3into-1 system were greatly reduced weight and improved cornering clearance from its compactness. If anyone knows of a current source for 3into-1 two-stroke pipes, let us know and we’ll run the address in a later service column.

Either Buchanan’s Frame Shop (629 E. Garvey Ave., Monterey Park, Calif. 91754, phone (213) 280-4003) or Meryl’s Pro Wheel (2899 B E. Coronado St., Anaheim, Calif. 92806, phone (714) 630-6815) could supply a suitable 16 in. rim and spokes, but installing a 16 in. wheel on the front of a GT750 isn’t a modification we’d recommend. The motorcycle companies have designed their new motorcycles around 16 in. fronts, and simply fitting a 16 in. wheel to a motorcycle designed to use a 19 in. may cause stability problems.

Head removal

I know you guys routinely remove the heads from motors that aren’t even through their first year, but how do you get one off a BMW R60/2 that’s been on for 14 years and 145,000 mi.?

Yes, I have all the head bolts off and I’ve even tried blowing it off with piston compression.

Dave Dexter Bellevue, Wash.

First make absolutely sure all nuts and bolts holding the head in place are out. That’s straightforward on your BMW, but you might be surprised at how many hours have been spent tugging and swearing at cylinder heads that wouldn’t come off until that hidden head bolt was removed. Next, spray oil on the cylinder-cylinder head junction and the studs passing through the head to help loosen parts that may have corroded together. After that, it’s time to apply force in increasing amounts until the head comes off.

We’d start with pulling on the cylinder head while striking it with a large, soft-faced hammer, taking care to hit on solid parts of the head casting and not on fragile fins. If that doesn’t work, escalate. Try driving wooden wedges between the cylinder and head, or find some way to use a pry bar and a wooden block to lever the head off. Again, be very careful to only exert force on thick sections of the head and cylinder or you’ll end up with bent or broken fins. If the head isn’t off after that, there’s still hope. You can make up a special fitting using an old spark plug that will allow the combustion chamber to be pressurized, either with air or hydraulic oil. High-pressure air can be found at almost any gas station, and a fire extinguisher service shop will have a hydraulic tester that will supply up to 600 psi pressure. Either of these should pop the head off, but if you use air, screw the head bolts in loosely to eliminate any chance of the head flying across the room.

Oil cooler need?

I read an ad for oil coolers that stated that engine oils work best between 160190° and that engine wear increases dramatically when oil temperature rises above 210°. However, in a March,

1982, Cycle World, you stated that oil coolers are more for looks than necessity, and that most bikes seldom if ever produce temperatures that would promote oil breakdown. A test of the Kawasaki GPz750 in the same issue stated that Kawasaki found in testing that power fell off as heat built up.

Would my KZ550 be better off with an oil cooler? I know that the answer depends on riding habits, but the facts •as stated are contradictory.

Glenn Toth

Los Angeles, Calif.

The oil cooler manufacturer may have overstated his case when he claimed that engine wear increases dramatically when the oil temperature is above 210°. That heat level may shorten oil life, but as long as you change oil at the suggested intervals, it won’t have much effect on engine life. High oil and engine temperatures may reduce engine power somewhat, but the example of the GPz750 confirms the point we make about oil coolers: manufacturers test new motorcycles to see if they’re adequately cooled, and if they’re not, they fix them so they are. In the case of the GPz750, it received an oil cooler that wasn’t required on the more mildly tuned KZ750.

Unless you run your motorcycle under unusual conditions, it doesn’t really need an oil cooler. If you do add an oil cooler for increased safety margin under extreme use, make sure you select an oil cooler equipped with a thermostat that routes the oil through the cooler only when the oil is hot. That will avoid overcooling during normal riding.

Valve seats vs. leaded petrol

I have recently moved to England with my 1981 GS650G in tow. Once here I was told that because of the lead content in the fuel that the valve seats, pistons, and rings should be replaced, or the engine would eventually be destroyed. Is this true?

Since my engine would have to be taken apart I was informed it would be a good idea to have new valves and cams installed, since they would improve the performance of the engine. Is there really any difference between U.S. and European hardware?

Richard P. Worsley

Oxfordshire, England

No. At one time valve seats and valves in motorcycle engines (and car engines) relied on lead as a lubricant, but almost all current U.S. model motorcycle engines can run on leaded or unleaded fuel. We know of no problem with running a engine designed for unleaded gasoline on leaded fuel except shorter spark plug life.

There are occasional differences between U.S. and European model motorcycles in carburetion and cam timing, but these changes are usually to comply with U.S. emission requirements, not because of different fuels. ®